A lawyer who stabbed his wife to death just days after she told him about her affair with a family friend yesterday told a jury about the moment he first lashed out at her with a 12cm (4.8in) kitchen knife.

Christopher Lumsden, then 51, told Manchester crown court that he had been in the bedroom of his home on the evening of March 16 last year when his wife, Alison, 53, just back from an evening out, went to thank him for turning on the electric blanket.

"She sat down at the dressing table," Mr Lumsden said. "I found myself getting out of bed. I grabbed the knife in the drawer next to me. I walked around the bed. She saw me and started to stand and turn. I bought the knife downwards. Everything went blank."

The next thing he remembered was dialling his sister's number and saying: "I think I've killed Alison."

Mr Lumsden, who attacked his wife five days after learning of her affair with family friend Roger Flint, cried frequently in the witness box. He said he had been "stunned and numbed" by news of the affair: "I had trouble believing what I heard. Alison lied to me, betrayed me with someone I regarded as a friend. She was abandoning me."

Mr Lumsden told the jury that problems had begun when his wife's HRT treatment made her "hypercritical", causing her to verbally attack him and her daughter. They became estranged and she moved into another bedroom at their home in Altrincham, Greater Manchester.

He said he had contemplated suicide in 2004 after being diagnosed with a muscle wasting illness and told he had as few as three years to live. He was a keen sportsman with a large circle of friends but gradually found himself unable to take part in social or sporting events.

"Suicide seemed the sensible thing to do," he said. "It wasn't dying that frightened me, it was living. I thought I could avoid the suffering of being in a wheelchair and deteriorating beyond that."

He had wanted his marriage to work, he said. "I told [Mrs Lumsden] I still loved her. I said I would do anything. I would change jobs, houses, towns. She said our marriage was over. There was no prospect of a reconciliation because I could no longer do the things she enjoyed doing. I was too dependent on her socially. She said I should get a life and date some people. Emotionally, I was all over the place."

On the night of the fatal attack he was "sinking into a greater and greater despair" and "the pressure of everything got too much" for him.

Under cross-examination by Charles Chruszcz, prosecuting, Mr Lumsden agreed that as a youth he had a "volcanic and uncontrollable temper". Mr Chruszcz also told the court that Mr Lumsden had given police two accounts of what happened. In the first, he said the knife was already in the bedroom. In another interview he said he took the knife from the kitchen on his way to bed.

Mr Chruszcz said he believed Mr Lumsden had intended kill his wife to which he replied: "I acknowledge that superficially it appears that way, but I had nothing in my mind at the time that said it."